Box jellyfish are best known as near-invisible marine killers and a scourge of tropical holidaymakers, but it seems they should also be known for their oddball eyes.

Zoologists have discovered that the jellyfish use a remarkably sophisticated eye to produce a blurred image. This helps the creatures by making sure the larger, more important objects in a jellyfish world do not get lost among smaller, less meaningful ones.

The jellyfish don't have a brain to deal with any incoming visual information; they rely instead on a simple ring of nerves to coordinate behaviour. Researchers think that the mass of imagery and light beaming into a box jellyfish's 24 eyes may provide the type of information the creature needs, without it having to filter or process any of these data.

Box jellyfish, also known as cubozoans owing to their cubic shape, are unusual in the jellyfish world because they are active predators. Unlike other species that drift along and eat whatever their tentacles ensnare, box jellyfish behave more like a fish, swimming towards interesting objects and skirting obstacles.

To help it perform these feats, a box jellyfish has a cluster of six eyes at each of its four corners. Dan-Eric Nilsson and colleagues at Sweden's Lund University decided to investigate these eyes, in the Caribbean species Tripedalia cystophora, to see exactly how they perform.

Into focus

The jellyfish has six eye clusters. Each contains four very simple eyes consisting of pigment-filled pits to catch light, and a pair of more complex, lensed eyes.

At just a tenth of a millimetre across, the lenses are made of material that has variable optical properties. The refractive index of the material, which indicates how much it bends light, is higher at the centre of the lens than at its edges, the researchers report in this week's Nature1.

This property helps a lens to focus all incoming light to a single point. Squid, octopuses and fish have similarly graded lenses to help them see sharp images, Nilsson says.

But in the jellyfish, this type of lens seems to serve the opposite purpose, ensuring that the image stays fuzzy. The jellyfish's retinas are placed behind the point at which the image is focused, meaning that they probably see a blur.

Blurred vision

This might make sense if the jellyfish use their eyes chiefly for navigation rather than spotting small prey, the researchers suggest. "It would help the animals to detect the large and stationary structures of their visual environment, but would leave unseen the plankton and small particles floating with the current," they point out.

A similar system is used by insects that use a blurry image of the horizon to navigate during flight, says Rüdiger Wehner, a zoologist at the University of Zurich, Switzerland. Seeing a general picture helps them to avoid making mistakes, he explains: "You don't want to see details of the skyline. It's better to blur."

Humans and other animals can make sense of a range of visual data, because they have brains that can filter and select the more important information. Nilsson and his team wonder if the jellyfish's finely tuned visual system makes up, to some extent, for their lack of such cognitive complexity.

They are now looking at a range of jellyfish species to see whether this information about their eyes gives insight into behaviour.